Our purposes are to determine the predominant sites of maximal expiratory flow limitation in the chronic and intermittently acute forms of airways obstruction and to examine the predominant location of acute changes in normal subjects with either bronchoconstrictor challenges or bronchodilator administration. The long-term goal is to assess the usefulness of location of obstruction in relation to the classification, prognosis and response to acute and prolonged therapy of obstructive airways syndrome.